Tape lettering systems employing a dry lettering printing process that mechanically transfers an impression of a character on a rotatable type disc from a dry film ribbon to an image carrying tape by means of a high pressure printing force are well known in the prior art, and are shown and described in U.S. Pat. Nos. 3,834,507; 4,243,333; and 4,402,619. An automated tape lettering machine employing this process is shown and described in U.S. Pat. No. 4,462,708. While each of these prior art machines is capable of generating high quality printing and lettering results, there is a need for a high speed tape printing apparatus capable of generating and modifying the appearance of the characters without the limitations imposed by the using a rotatable type disc.
In an apparatus or system for dry lettering of the type described above, the operator is required to physically change the type disc each time a new size or font is desired that is not available on the current type disc. As the number of sizes and fonts available on a type disc is directly limited by the physical size of the type disc, printing a sequence of characters of varying sizes and fonts is prohibitively time consuming and expensive, requiring a large number of type discs to provide the varying sizes and fonts of characters that may be desired. In addition, because an operator is generally required to change type discs for a change in size or font, the performance advantages of the automated tape lettering machine described in U.S. Pat. No. 4,462,708 can not be achieved when the sequence of characters to be printed includes type sizes or fonts that are not all available on a single type disc.
Column-oriented thermal transfer printing devices also exist in which an image of a desired character is formed on a strip of image carrying tape by transferring ink or other color from a color carrying ribbon to the tape as a result of the localized application of heat and small amount of pressure. A typical thermal transfer device of this type is described in U.S. Pat. No. 4,666,319. Another thermal transfer device presently available employs a thermal print head for transferring images from a strip of ribbon to a strip of tape and has a cooperating tape-ribbon cartridge for providing a supply of tape and ribbon to the device. While such devices are useful for printing smaller point size characters, the fonts for such characters are represented using a dot matrix array format that limits the number of sizes and fonts of print that are available. The limitations of this method of representation make it impractical to perform in real-time any type of manipulations to vary the size, arrangement, sequence, orientation or attributes of the characters to be printed.
In a dot-matrix thermal printer of the type described above, fonts are stored in a dot-matrix array format with each pixel to be printed represented in the font data, for instance a 5.times.7 dot matrix array. This type of representation of the font is not practical when the dimension of the array begin to approach the pixel resolution necessary for high quality lettering in point sizes larger than 14 points. For example, the number of pixels that must be stored for each character or image in a 5.times.7 dot-matrix array would be a maximum of 35 pixels. If, however, the dot-matrix array needed to represent characters or images having 300.times.400 dot-matrix array, each character would require a maximum of 120,000 pixels to be stored. With more than 150 characters and other images present in a typical font character set, requiring 120,000 pixels or bits of information to be stored for each character would mean that 10,000,000 bits or 1.25 million bytes of information would be necessary to represent an entire character set for a font of a single type and size.
In other pixel-based printer or display applications, for example laser printers, video displays and photo typesetters, font information is sometimes stored in an outline format, rather than in a dot-matrix array format. By storing only the outline of a character in the font data, the system has all of the information necessary to print or display that character by a process known as "rasterization", generating the outline of the character and filling in the area inside the outline. Outline information for a particular character or image may be stored as a set of all points representing the outline of the character, U.S. Pat. No. 3,305,841, a set of vectors representing the outline of the character, U.S. Pat. No. 4,298,945, or a set of arcs or curves representing the outline of the character, U.S. Pat. Nos. 4,542,412, and 4,675,833. In all of the rasterization systems described above, rasterization occurs in a non real-time environment because the printer or display is generating a direct bit map of the entire page or line to be printed or displayed, rather than printing or displaying a single column of pixels as they are generated. While such systems are useful for printing or displaying entire lines or pages of characters, they necessarily require a large processing capability and sufficient amounts of memory to store an entire bit map for the line or page of output as it is being generated prior to display, capabilities that are not necessary in the type of printing or display apparatus that outputs characters as they are generated.
Although the above devices may be satisfactory for various uses and applications, they do not provide for the ability to represent a wide variety of type sizes and fonts and also allow for the real-time manipulation of the size, arrangement, sequence, orientation, or attributes of characters or images in the type of printing or display apparatus that outputs characters as they are generated. Accordingly, there is a continuing need for improvements in the real-time rasterization systems associated with column-oriented printing and display apparatus and, in particular, in the real-time rasterization systems associated with thermal transfer tape lettering systems capable of performing real-time rasterization and manipulation of the fonts for characters to be printed on a strip of tape.